Electrical Bad ECM?

Discussion in 'Fox 5.0 Mustang Tech' started by tca7291, Apr 13, 2013.

  1. Hey fellas, I've been working on my buddies 1990 GT and I think his ecm is shot, but just wanted to check with y'all before I condemn it. It will start up, and will run fine for a minute or so, but will lean out and stall eventually. When it messes up the fuel pump relay, and the WOT A/C clutch cycle relay start tripping out pretty bad. They will start clicking on and off very fast and erratically. I checked the ecm power relay and it stays on as it should. Do any of you guys have any other suggestions, or am I more or less correct on saying that the ecm is fried? Thanks for any help!
  2. Have you tried pulling codes yet?
  3. I am by no means an expert, but when it comes to electrical, I have learned that good grounds mean everything.
    Have you checked that all ground straps and cables, as well as the ground to the ECM box itself are clean and tight?
    Just a thought before you go and drop a hundred dollar bill on an ECM.

    Also, the A/C and fuel pump relays are tied into the TPS and the key switch.
    2 other places you might check for bad wires.
  4. Yes noobz, we have pulled codes several times to no avail, the computer doesn't throw any stored or in the KOEO test, and the poor pony won't run long enough to do a KOER test. Ratio, we haven't really gone through all the grounds yet, but that is a great idea, and it should be easy considering the car has no interior right now, and I did check all the wiring at the ignition switch. I don't know if this would make a difference, but it started acting up like this after sitting outside parked for about two years. Perhaps I should try to call jrichker in on this!
  5. What I might try to do to eliminate the ignition switch and wiring from the equation is make yourself a couple temporary power and grounds directly to the EEC from the battery.

    If it quits again, then you're reasonably certain that the EEC needs help. If it doesn't, you know you have a wiring problem.

    Additionally... When it quits, have you measured voltage to the EEC?
  6. Thanks noobz, I will have to try running some jumpers to the ecm, and also measure voltage. I did make a jumper last night to the fuel pump relay, I was thinking that she was leaning out and dying because of the fuel pump being interrupted, but that didn't help at all. Thanks again for the ideas!
  7. Here is a video that I just took to give you guys a better idea of exactly what I am talking about. That funny noise at about :30 is the WOT clutch cycle relay pulsating on and off like I said.

  8. Can't view the video, says it's private...
  9. Video should be fixed, but if not, this should be another link.

  10. No code 11 (two flashes when the computer dumps the codes)? A perfectly functioning system will always get a code 11, it is the everything is OK code. The 11 code is computer passed its internal self test.
    If you don't get an 11, you have computer or wiring problems.
    Please check and repost.
  11. Totally forgot about 11.:bang:Nope, no code 11. jrichker, you're the best!!! I pulled codes again earlier this afternoon, and all that was displayed was a 85, and then a 90 something, and while I had the computer flashing the codes, it went haywire again with the relays and the cheap little code reader (it only has an LED and a beep) started tripping out just like the relays have done. I'm starting to be pretty confident that the ecm is fried.
  12. Were your results from an engine running dump or key on engine not running dump?
  13. Key on engine off dump.
  14. The code 85 is valid, but I can't find a code 90 with the engine not running.

    Code 85 - CANP solenoid - The Carbon Canister solenoid is inoperative or missing. Check vacuum lines for leaks and cracks. Check electrical wiring for loose connections, damaged wiring and insulation. Check solenoid valve operation by grounding the gray/yellow wire to the solenoid and blowing through it.
    The computer provides the ground for the solenoid. The red wire to the solenoid is always energized any time the ignition switch is in the run position.

    Charcoal canister plumbing - one 3/8" tube from the bottom of the upper manifold to the rubber hose. Rubber hose connects to one side of the canister solenoid valve. Other side of the solenoid valve connects to one side of the canister. The other side of the canister connects to a rubber hose that connects to a line that goes all the way back to the gas tank. There is an electrical connector coming from the passenger side injector harness near #1 injector that plugs into the canister solenoid valve. It's purpose is to vent the gas tank. The solenoid valve opens at cruse to provide some extra fuel. The canister is normally mounted on the passenger side frame rail near the smog pump pulley.


    It does not weigh but a pound or so and helps richen up the cruse mixture. It draws no HP & keeps the car from smelling like gasoline in a closed garage. So with all these good things and no bad ones, why not hook it up & use it?

    The purge valve solenoid connector is a dangling wire that is near the ECT sensor and oil filler on the passenger side rocker cover. The actual solenoid valve is down next to the carbon canister. There is about 12"-16" of wire that runs parallel to the canister vent hose that comes off the bottom side of the upper intake manifold. That hose connects one port of the solenoid valve; the other port connects to the carbon canister.

    The purge valve solenoid should be available at your local auto parts store.

    Purge valve solenoid:

    The carbon canister is normally mounted on the passenger side frame rail near the smog pump pulley.
    Carbon Canister:
  15. Thanks for the info on the purge/emission stuff, but we were not too concerned about that stuff being inoperable at the time. The car ran great about 2 1/2 years ago, I drove it from MD down to NC, about a 300 mile trip non stop, and she ran fine for a while after that. Got to my bud's house and he parked it, left her sitting for a long time (probably about 6 months) without even starting her up, and obviously the battery went dead. Wouldn't take a charge, so replaced battery and started having this issue after that. Been going on sense, and one more thing I noticed last weekend is that it seems like she will run ok when cold, run for about a minute or two, and then it will just start messing up and keep messing up every time you try to start it. IIRC, KOEO test should flash the 11 code, correct?
  16. Yes, code 11 means that the computer has passed its self test.
  17. Sorry man, I must of confused you a little, we didn't get an 11 from the KOEO dump. The 'puter went through it's normal tests, and never flashed an 11. Just went straight to the 85 after the MIL light stopped flickering as it has always done during the test.

    So I guess my final question would be, if the computer has failed it's own self test, does that mean 100% for sure that it is toast? Thanks again!
  18. Computer will not go into diagnostic mode on 86-90 model 5.0 Mustangs

    Disconnect the battery positive terminal before making any resistance checks.
    The voltage drop in the ground cable will cause incorrect resistance readings.

    How it is supposed to work:
    The black/white wire (pin 46) is signal ground for the computer. It provides a dedicated ground for the EGR, Baro, ACT, ECT, & TPS sensors as well as the ground to put the computer into self test mode. If this ground is bad, none of the sensors mentioned will work properly. That will severely affect the car's performance. You will have hard starting, low power and drivability problems. Since it is a dedicated ground, it passes through the computer on its way to the computer main power ground that terminates at the battery pigtail ground. It should read less than 1.5 ohms when measured from anyplace on the engine harness with the battery pigtail ground as the other reference point for the ohmmeter probe.

    What sometimes happens is that the test connector black/white wire gets jumpered to power which either burns up the wiring or burns the trace off the pc board inside the computer. That trace connects pins 46 to pins 40 & 60.

    The STI (Self Test Input ) is jumpered to ground to put the computer into test mode. Jumpering it to power can produce unknown results, including damage to the computer. The ohm test simply verifies that there are no breaks in the wiring between the test connector and the computer input.

    How to test the wiring :
    With the power off, measure the resistance between the computer test ground (black/white wire) on the self test connector and battery ground. You should see less than 1.5 ohms.


    If that check fails, remove the passenger side kick panel and disconnect the computer connector. There is a 10 MM bolt that holds it in place. Measure the resistance between the black/white wire and pin 46 on the computer wiring connector: it should be less than 1.5 ohms. More that 1.5 ohms is a wiring problem. If it reads 1.5 ohms or less, then the computer is suspect. On the computer, measure the resistance between pin 46 and pins 40 & 60: it should be less than 1.5 ohms. More that that and the computer’s internal ground has failed, and the computer needs to be repaired or replaced.

    See http://www.stangnet.com/mustang-forums/749974-computer-issue.html#post7490537 for Joel5.0’s fix for the computer internal signal ground.

    If the first ground check was good, there are other wires to check. Measure the resistance between the STI computer self test connector (red/white wire) and pin 48 on the computer main connector: it should be less than 1.5 ohms. More that 1.5 ohms is a wiring problem

    The following is a view from the computer side of the computer wiring connector: it is for an A9L, A9P computer.


    Diagram courtesy of Tmoss & Stang&2birds

    Check out the diagram and notice all the places the black/white wire goes. Almost every sensor on the engine except the MAF is connected to it.


    See the following website for some help from Tmoss (diagram designer) & Stang&2Birds
    (website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine

    See the graphic for the 10 pin connector circuit layout.